The art of coating surfaces, as for example, painting a metal surface to impart a protective or decorative coating onto the metal surface, is an old and widely practiced art. The techniques for applying compositions which contain film-forming resins, both natural and synthetic and which may be pigmented, otherwise colored or opaqued, are varied, depending upon the compositions employed, the requirements of production and the skill of the appliers. Because most commercial coating compositions require rapid drying, solvents or vehicles employed for dissolving or suspending the film-forming materials, pigments and coloring or opaquing agents are normally highly volatile and, according to present day techniques, are lost to the ambient atmosphere. This has led to the employment of inexpensive materials such as toluene, V&PM naphthas and the like. However, because these solvents are flammable, little or no effort has been made to recover them and reuse them; as a matter of fact, some processes burn the vapors given off during the drying step. Recent demands for safer materials from the fire hazard standpoint has prompted exploration into the use of less flammable liquids such as the halogenated hydrocarbons. These materials are relatively expensive and therefore must be recovered if they are to be used industrially. Therefore, the use of these solvents has not gained any prominence in industrial coating processes.
The aforedescribed properties of the solvent vehicles, particularly their high volatility, contribute to uneven surfaces and the attendant loss of gloss of the coating. In some instances, a haze or "blush" develops as a result of high volatility and rapid evaporation, particularly on a moist or humid day. To overcome the loss of gloss and to produce a more uniform surface with greater reflective properties, the technique of the thermal reflow, i.e., heating the thermoplastic coating to its softening point to permit it to level itself, is practiced as a normal phase of the required "bake." The "bake" is required to drive off residual solvent and, in some instances, to cause a reaction of the film-forming material, e.g., either chemical cross-linking, or a reaction somewhat similar to that achieved with driers in conventional paints and lacquers. Solutions to the many problems have been suggested, and in small scale operations, one answer again comes to the foreground - use of the less flammable solvents and blends of these solvents to obtain desirable drying times. This, as discussed above, has not become widespread because of its cost when employed in present-day techniques.
It is an object of the present invention to provide a process for coating surfaces with desirable film-forming materials, which may be pigmented, colored, opaqued or unpigmented, by application from substantially nonflammable organic solvents in an ecologically satisfactory manner.
A further object of the present invention is to provide a process for coating surfaces utilizing a nonflammable organic solvent and recovering the solvent for reuse thereby to provide an economically satisfactory process.
A still further object of the present invention is to provide a process for coating surfaces which requires no release of solvents to the atmosphere during "drying" and if a thermal bake or cure is necessary or desirable, a process which produces low residual solvent content in the surface coating thereby reducing solvent released to the atmosphere during bake or cure, thus providing an ecologically satisfactory process.
Another object of the present invention is to provide a process for coating which permits the preparation of coatings of up to 2 or more mils in thickness in a single application of coating composition which coatings have a high gloss, level coat in time periods equal to or less than that time normally required to obtain equivalent coat thickness using the conventional solvents and coating techniques.
A still further object is to provide a process which permits a primer coating to be applied followed by one or more top coats without intermediate baking or curing of the primer coat or the top coats.